1. Field of the Invention
The present invention relates to an improvement over optical glasses having high dispersions.
2. Description of the Prior Art
There are known and used various optical glasses having high dispersions. Among them, those optical glasses are of importance which are generally called flint glass, dense flint glass, dense barium flint glass etc. and which comprise silicic anhydride or boric anhydride as network forming oxide, alkali metal or alkaline earth metal oxide, zinc oxide, lead oxide, titanium oxide and the like as network modifying oxide or aluminium oxide as intermediate oxide. This class of optical glasses is widely used as an indispensable material for the preparation of optical system components. However, their physical properties are not always satisfactory, in particular, with respect to light transmittivity. This deficiency is found especially in those optical glasses which are high in refractive index and low in the contents of sillicic anhydride. With the increasing popularity of color photography, the yellowing of glass which is often observed in optical glasses has become a much more important problem than before. Furthermore, the polish processing technique of optical glass has noticeably progressed recently, which in turn subjects the optical glass to a severer processing condition. Also, the increasing use of reflection reducing optical coating film requires the optical glass to have a surface of excellent stability against corrosion and deterioration. For these reasons, the chemical durability of optical glasses has become much more important than before.